1. Field of the Invention
A tool for forming a threaded hole in a plastically deformable metal.
2. Description of the Prior Art
A drill bit of the prior art is a well-known rectilinear tool having a cutting tip to initially penetrate into a solid body. The shanks of the bits extend back from the cutting tips. There are two major types of bits, namely, a spade bit and a twist bit. In a twist bit the shank is provided with spiral or helical grooves having peripheral cutting edges. The bit generally is cylindrical. In a spade bit the shank usually is rectangular and approximately flat. The back end of the bit is inserted into the chuck of a suitable means to impart rotary motion about the central longitudinal axis to the bit. Thus, the bit, when mounted in the chuck of a drill press, hand-operated drill, etc., or when mounted in any suitable means to concomitantly impart rotary motion and to exert inward cutting force against a solid body, serves to cut and drill a smooth-walled cylindrical hole into the solid body. The diameter of the hole is, of course, equal to the diameter of the shank of the bit.
A cutting tap of the prior art is a tool which is inserted (by axial rotation and axial force) into a previously drilled smooth-walled generally cylindrical hole in metal. A cutting tap is a rectilinear generally cylindrical tool. However, the function of a cutting tap is vastly different from that of a drill. The cutting tap has annular spaced cutting teeth along its shank, arrayed in rows which extend longitudinally in parallel along the perimetral surface of the tool. The rows of teeth are separated by grooves. The mode of using a cutting tap is similar to that employed in using a drill, except that the minimum diameter of the cutting tap is of slightly greater diameter than that of the pre-existing hole and, as it is manually forced downward into the hole, by a combination of downward force and twisting (rotary motion about its longitudinal axis), the tap cuts metal from the wall of the hole and makes a spiral thread in the wall of the hole.
A coining tap, as its name implies, coins a screw thread in a pre-existing hole. Its minimum diameter is smaller than that of the pre-existing hole.
An existing "drap", namely a combined drill and tap, drills and cuts a thread, and does not coin or form a thread.
Pertinent prior art relative to bits and taps includes U.S. Pat. Nos. 3,195,156 and 3,812,698.